A conventional article of athletic footwear includes two primary elements, an upper and a sole structure. The upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a variety of activities such as walking and running. The upper provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure is secured to a lower surface of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces, the sole structure may provide traction and control foot motions.
Current sole structure designs, however, provide a large portion of the overall weight of the athletic footwear. Heavier athletic footwear directly impacts comfort of the wearer and may decrease performance during a multitude of activities such as running and walking Current sole structure designs have focused on using lighter weight materials in the design of the sole structure to reduce the overall weight of the athletic footwear.
The sole structure of athletic footwear generally exhibits a layered configuration that includes a comfort-enhancing insole, a resilient midsole formed from a polymer foam material, and a ground-contacting outsole that provides both abrasion-resistance and traction. The midsole is the primary sole structure element that attenuates ground reaction forces and controls foot motions. Suitable polymer foam materials for the midsole include ethylvinylacetate or polyurethane that compress resiliently under an applied load to attenuate ground reaction forces. Conventional polymer foam materials are resiliently compressible, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas. The polymer foam materials of the midsole also may absorb energy when compressed during ambulatory activities.
The midsole may be formed from a unitary element of polymer foam that extends throughout the length and width of the footwear. With the exception of a thickness differential between the heel and forefoot areas of the footwear, such a midsole exhibits substantially uniform properties in each area of the sole structure.